The Photoelectric Effect

At the beginning of the 19th century, scientists were arguing whether light was a wave or a particle. By the end of the 19th century, all were agreed that light was a wave.

So it was surprising when Albert Einstein showed in 1905 that in some respects, the particle theory was right after all.

Particles of light were needed to explain the photoelectric effect, where 'chunks' of light, called photons, knock electrons off a sheet of metal.

Einstein showed that photons were absorbed in one go. If the light did not have enough energy to knock an electron off instantly, it would never be able to knock an electron off the sheet of metal. Light could not be absorbed slowly, like a wave, as scientists had previously thought.

Today the photoelectric effect is used in solar cells to create useful currents. The sheets of metal in a solar cell are arranged so that when light knocks an electron off, a circuit is completed and a current is generated.